Where is the evidence that massive genetic changes occurred among the indigenous Scots from the first century on? Germanic, Nordic and Atlantic Facade influences certainly were already part of the Scottish genome at the time. Increased Nordic, Irish and English contributions (in the main) from, say, the 9th century onwards, simply enhanced elements that had long formed the Scottish genetic substrata.
Why would the changes have to be "massive" to reduce the frequency of red hair between the 1st century and the 21st century? Red hair is a recessive trait.
What of the various peoples who have had a cumulative impact on the Scottish genetic landscape since the 1st century? Roman soldiers from all over the empire served in the outposts along Hadrian's Wall. What of the nearly 6,000 Sarmatians stationed in northern Britain? What of the Vikings, Normans, French, Flemish, Jews and Gypsies who are known to have settled in Scotland?
Nearly 2,000 years have passed since Tacitus wrote his description of the 1st century Caledonians. It is ridiculous to argue that because modern Scots have red hair at a frequency of about 5% (claimed as the result of a study I have not seen) that Tacitus must have been absolutely wrong and is therefore unreliable.
Tacitus' comments need to be taken at face value for what they were: anecdotal observations. One need not believe he was asserting that absolutely every last man, woman and child of the Caledonians had red hair and large limbs, but merely that red hair and large limbs were common enough traits among them to be worthy of note.
The conclusion that Tacitus draws of a connection to Germany is simply his own. Apparently red hair and large limbs were also common enough to be likewise worthy of note among the tribes of Germania (whether Germanic speaking or Celtic speaking is unclear) with which the Romans were most familiar.
The value of Tacitus' comments is that they reflect contemporary observations of some of the physical traits of 1st century people. It is not necessary to make inferences that take them to extremes.
This thread was begun by a guy of Scottish descent who apparently got some kind of results connecting him to Germanic people. He is L21+, a common enough value in Scotland but also in Germany, especially southwestern Germany. I posted the quote from Tacitus as an interesting historical observation and something worthy of consideration.
Why that should cause such a furor is beyond me.
